1. Field of the Invention
The invention relates to a method for forming a P-type gallium nitride.
2. Description of the Related Art
Currently, in all materials used for manufacturing luminescent devices which can emit blue, purple or ultraviolet light rays, gallium nitride has the best potential, because the gallium nitride has a direct energy gap structure and its energy gap is about 3.39 eV at a room temperature. In the prior art, a hetero-epitaxial growing process performed on a sapphire substrate is used to form single-crystal gallium nitride. However, since there are great differences between the gallium nitride and sapphire substrate in lattice match and coefficient of thermal expansion, it is very difficult to grow gallium nitride of high quality and high planarization. Nevertheless, this problem can be overcome by depositing an AlN layer to function as a buffer layer on the sapphire substrate before the gallium nitride is grown. Therefore, the planarization, crystal quality and electric properties of the gallium nitride layer can be improved.
In order to further improve the efficiency of an LED which emits blue light, and manufacture an LED with gallium nitride, development and research of P-type gallium nitride is urgently necessary. Growing gallium nitride doped with magnesium (Mg) is disclosed in Jpn. J. Appl. Phys. Vol. 28, No. 12, Dec. 1989, pp. L2112-L2114 by Hiroshi Amano, et al., wherein a low-energy electron beam process is performed after the magnesium-doped gallium nitride is grown, so that the formed magnesium-doped gallium nitride has a hole concentration on the order of 2.times.10.sup.16 cm.sup.-3, a hole mobility on the order of 8cm.sup.2 /Vxs, and a resistivity on the order of 35 .OMEGA..times.cm. However, P-type gallium nitride having the above-mentioned data can not be sufficiently utilized to manufacture a blue light ray-emitting and high power LED. To resolve this problem, annealing magnesium-doped gallium nitride at a temperature on the order of 700.degree. C. is disclosed in Jpn. J. Appl. Phys. Vol. 31 (1992), Pt. 2, No. 2B, pp. L139-L142 by Shuji Nakamura et al., wherein hydrogen can be dissociated from the Mg--H to activate the magnesium (acceptor), such that the formed P-type gallium nitride has a low impedance, and wherein this P-type gallium nitride has a resistivity on the order of 2 .OMEGA..times.cm, a hole concentration on the order of 3.times.10.sup.17 cm.sup.-3, and a hole mobility on the order of 10 cm.sup.2 /V-s.
Generally, the annealing temperature should be kept in a range of 700.degree..about.800.degree. C. If the temperature is below 700.degree. C., the Mg--H is not easily decomposed completely. Furthermore, if the hydrogen atoms can not be completely dissociated from the gallium nitride, once the temperature is lowered, the dissociated hydrogen atoms can combine with the magnesium into Mg--H immediately. As a result, the magnesium can not be activated and the resistance is increased. On the other hand, if the temperature is higher than 700.degree.C., the nitrogen atoms inside the gallium nitride are dissociated, resulting in V.sub.N gap defects.